Imaging based on electromagnetic radiation can be implemented, for instance, as so-called full-field imaging or scanning imaging with a narrow sensor. As the manufacturing costs of digital sensors, with current technology, grow exponentially as a function of sensor area, there is a tendency to employ scanning imaging whenever possible. In scanning imaging the area to be imaged is scanned with a narrow fan-beam, typically with a beam considerably narrower than the object to be imaged, and the scanning movement of the beam is followed by a narrow sensor from which image information is continuously read out during the imaging scan.
Semiconductor sensors commonly used in digital imaging have a basic structure in which small picture elements, i.e. pixels, form larger radiation-sensitive areas. Sensors of traditional technology are able to detect mainly wavelengths of visible light only, which means that x-ray quanta, for example, must be converted to light photons which are in turn converted into electric signal that forms the image information of the pixels. In x-ray sensors of more modern technology, the so-called direct detection sensors, the radiation arriving in the area of picture elements is absorbed in a medium in which it is directly converted into electron-hole pairs, in other words, into electrically detectable charges. The medium can be biased (photoelectric) semiconductor material, such as Ge, Si, Se, GaAs, Hgl, CdTe, CdZnTe or Pbl, and when an electric field is arranged over it, each of the electron-hole pairs produced by radiation quanta can be collected within the area of its own pixel. Such a sensor, utilizing collimation by an electric field and direct detection, enables very high quantum efficiency (dqe) without sacrificing resolution, as the material layer detecting radiation can be arranged sufficiently thick so as to absorb all the radiation quanta that enter it, without the charge generated thereby spreading to the area of adjacent pixels. Electric information of a pixel electrode can be detected either by measuring the amount of charge accumulated into a pixel in a time unit or, by counting each quantum absorbed into a pixel area discretely, i.e. using e.g. technology disclosed in WO 98/16853, according to which each charge impulse generated upon absorption of a radiation quantum increments a counter. When this detection method based on so-called photon counting is used, each of the quanta of different energy levels are counted individually, instead of first accumulating the charge generated thereby and subsequently measuring the magnitude of the accumulated charge. Using the photon counting method, the contrast of an image produced is remarkably improved.
Typically, pixels of the digital sensors have equal vertical and horizontal dimensions, the pixels being arranged such that they are evenly distributed on the active area of the sensor. When scanning imaging is implemented by transferring image information from one pixel to another as pixel-sized units, the point being imaged on each pixel “swings” in the scanning direction as a function of the effective pixel size used. In these typical prior art sensors, resolution in scanning imaging is clearly poorer in the scanning direction than in the direction perpendicular to it. In practice it has been found to be about half of the perpendicular resolution.
A seemingly natural solution to improve resolution in scanning direction would be reduction of pixel size, for instance, halving the size. However, that would quadruple the amount of read-out electronics required for the sensor and the amount of image information produced. Also, even though resolution in the scanning direction could be improved in this manner to the desired level, resolution in the perpendicular direction would still be twice better than in the scanning direction.
In principle, the pixel arrangement could be implemented densified closer only in the scanning direction, in other words, by reducing dimension of the pixels only in the scanning direction, but also in this case the amount of read-out electronics and the amount of image information would be increased respectively.